mac80211: fix rx->key NULL dereference during mic failure

Sometimes when reporting a MIC failure rx->key may be unset. This
code path is hit when receiving a packet meant for a multicast
address, and decryption is performed in HW.

Fortunately, the failing key_idx is not used for anything up to
(and including) usermode, so we allow ourselves to drop it on the
way up when a key cannot be retrieved.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/include/net/cfg80211.h b/include/net/cfg80211.h
index 0589f55..396e8fc 100644
--- a/include/net/cfg80211.h
+++ b/include/net/cfg80211.h
@@ -2688,7 +2688,7 @@
  * @dev: network device
  * @addr: The source MAC address of the frame
  * @key_type: The key type that the received frame used
- * @key_id: Key identifier (0..3)
+ * @key_id: Key identifier (0..3). Can be -1 if missing.
  * @tsc: The TSC value of the frame that generated the MIC failure (6 octets)
  * @gfp: allocation flags
  *